Chapter 7

Chapter 7
Expressions and
Assignment Statements
Chapter 7 Topics
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Introduction
Arithmetic Expressions
Overloaded Operators
Type Conversions
Relational and Boolean Expressions
Short-Circuit Evaluation
Assignment Statements
Mixed-Mode Assignment
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Introduction
• Expressions are the fundamental means of
specifying computations in a programming
language
• To understand expression evaluation, need to
be familiar with the orders of operator and
operand evaluation
• Essence of imperative languages is dominant
role of assignment statements
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Arithmetic Expressions
• Arithmetic evaluation was one of the
motivations for the development of the first
programming languages
• Arithmetic expressions consist of operators,
operands, parentheses, and function calls
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Arithmetic Expressions: Design Issues
• Design issues for arithmetic expressions
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Operator precedence rules?
Operator associativity rules?
Order of operand evaluation?
Operand evaluation side effects?
Operator overloading?
Type mixing in expressions?
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Arithmetic Expressions: Operators
• A unary operator has one operand
• A binary operator has two operands
• A ternary operator has three operands
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Arithmetic Expressions: Operator Precedence
Rules
• The operator precedence rules for
expression evaluation define the order in
which “adjacent” operators of different
precedence levels are evaluated
• Typical precedence levels
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parentheses
unary operators
** (if the language supports it)
*, /
+, 1-7
Arithmetic Expressions: Operator Associativity
Rule
• The operator associativity rules for expression evaluation
define the order in which adjacent operators with the same
precedence level are evaluated
• Typical associativity rules
– Left to right, except **, which is right to left
– Sometimes unary operators associate right to left (e.g., in FORTRAN)
• APL is different; all operators have equal precedence and all
operators associate right to left
• Precedence and associativity rules can be overriden with
parentheses
1-8
Expressions in Ruby and Scheme
• Ruby
– All arithmetic, relational, and assignment operators,
as well as array indexing, shifts, and bit-wise logic
operators, are implemented as methods
- One result of this is that these operators can all
be overriden by application programs
• Scheme (and Common LISP)
- All arithmetic and logic operations are by explicitly
called subprograms
- a + b * c is coded as (+ a (* b c))
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Arithmetic Expressions: Conditional Expressions
• Conditional Expressions
– C-based languages (e.g., C, C++)
– An example:
average = (count == 0) ? 0 : sum / count
– Evaluates as if written as follows:
if (count == 0)
average = 0
else
average = sum /count
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Arithmetic Expressions: Operand Evaluation
Order
• Operand evaluation order
1. Variables: fetch the value from memory
2. Constants: sometimes a fetch from memory;
sometimes the constant is in the machine
language instruction
3. Parenthesized expressions: evaluate all operands
and operators first
4. The most interesting case is when an operand is
a function call
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Arithmetic Expressions: Potentials for Side
Effects
• Functional side effects: when a function changes a two-way
parameter or a non-local variable
• Problem with functional side effects:
– When a function referenced in an expression alters another operand
of the expression; e.g., for a parameter change:
a = 10;
/* assume that fun changes its parameter */
b = a + fun(&a);
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Functional Side Effects
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Two possible solutions to the problem
1.
Write the language definition to disallow functional side effects
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2.
No two-way parameters in functions
No non-local references in functions
Advantage: it works!
Disadvantage: inflexibility of one-way parameters and lack of nonlocal references
Write the language definition to demand that operand evaluation
order be fixed
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Disadvantage: limits some compiler optimizations
Java requires that operands appear to be evaluated in left-to-right
order
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Referential Transparency
• A program has the property of referential
transparency if any two expressions in the
program that have the same value can be
substituted for one another anywhere in the
program, without affecting the action of the
program
result1 = (fun(a) + b) / (fun(a) – c);
temp = fun(a);
result2 = (temp + b) / (temp – c);
If fun has no side effects, result1 = result2
Otherwise, not, and referential transparency is violated
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Referential Transparency (continued)
• Advantage of referential transparency
– Semantics of a program is much easier to understand
if it has referential transparency
• Because they do not have variables, programs in
pure functional languages are referentially
transparent
– Functions cannot have state, which would be stored in
local variables
– If a function uses an outside value, it must be a
constant (there are no variables). So, the value of a
function depends only on its parameters
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Overloaded Operators
• Use of an operator for more than one purpose
is called operator overloading
• Some are common (e.g., + for int and
float)
• Some are potential trouble (e.g., * in C and
C++)
– Loss of compiler error detection (omission of an
operand should be a detectable error)
– Some loss of readability
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Overloaded Operators (continued)
• C++, C#, and F# allow user-defined overloaded
operators
– When sensibly used, such operators can be an aid
to readability (avoid method calls, expressions
appear natural)
– Potential problems:
• Users can define nonsense operations
• Readability may suffer, even when the operators make
sense
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Type Conversions
• A narrowing conversion is one that converts
an object to a type that cannot include all of
the values of the original type e.g., float to int
• A widening conversion is one in which an
object is converted to a type that can include
at least approximations to all of the values of
the original type
e.g., int to
float
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Type Conversions: Mixed Mode
• A mixed-mode expression is one that has operands of
different types
• A coercion is an implicit type conversion
• Disadvantage of coercions:
– They decrease in the type error detection ability of the compiler
• In most languages, all numeric types are coerced in
expressions, using widening conversions
• In Ada, there are virtually no coercions in expressions
• In ML and F#, there are no coercions in expressions
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Explicit Type Conversions
• Called casting in C-based languages
• Examples
– C: (int)angle
– F#: float(sum)
Note that F#’s syntax is similar to that of function
calls
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Errors in Expressions
• Causes
– Inherent limitations of arithmetic
e.g., division by zero
– Limitations of computer arithmetic
overflow
• Often ignored by the run-time system
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e.g.
Relational and Boolean Expressions
• Relational Expressions
– Use relational operators and operands of various
types
– Evaluate to some Boolean representation
– Operator symbols used vary somewhat among
languages (!=, /=, ~=, .NE., <>, #)
• JavaScript and PHP have two additional relational
operator, === and !==
- Similar to their cousins, == and !=, except that they
do not coerce their operands
– Ruby uses == for equality relation operator that uses
coercions and eql? for those that do not
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Relational and Boolean Expressions
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Relational and Boolean Expressions
• Boolean Expressions
– Operands are Boolean and the result is Boolean
– Example operators
• C89 has no Boolean type--it uses int type with 0
for false and nonzero for true
• One odd characteristic of C’s expressions:
a <
b < c is a legal expression, but the result is not
what you might expect:
– Left operator is evaluated, producing 0 or 1
– The evaluation result is then compared with the third
operand (i.e., c)
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Short Circuit Evaluation
• An expression in which the result is determined
without evaluating all of the operands and/or
operators
• Example: (13 * a) * (b / 13 – 1)
If a is zero, there is no need to evaluate (b
1)
/13 -
• Problem with non-short-circuit evaluation
index = 0;
while (index <= length) && (LIST[index] != value)
index++;
– When index=length, LIST[index] will cause an indexing
problem (assuming LIST is length - 1 long)
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Short Circuit Evaluation (continued)
• C, C++, and Java: use short-circuit evaluation for the usual
Boolean operators (&& and ||), but also provide bitwise
Boolean operators that are not short circuit (& and |)
• All logic operators in Ruby, Perl, ML, F#, and Python are shortcircuit evaluated
• Ada: programmer can specify either (short-circuit is specified
with and then and or else)
• Short-circuit evaluation exposes the potential problem of side
effects in expressions
e.g. (a > b) || (b++ / 3)
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Assignment Statements
• The general syntax
<target_var> <assign_operator> <expression>
• The assignment operator
= Fortran, BASIC, the C-based languages
:= Ada
• = can be bad when it is overloaded for the
relational operator for equality (that’s why the
C-based languages use == as the relational
operator)
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Assignment Statements: Conditional Targets
• Conditional targets (Perl)
($flag ? $total : $subtotal) = 0
Which is equivalent to
if ($flag){
$total = 0
} else {
$subtotal = 0
}
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Assignment Statements: Compound Assignment
Operators
• A shorthand method of specifying a
commonly needed form of assignment
• Introduced in ALGOL; adopted by C and the Cbased languaes
– Example
a = a + b
can be written as
a += b
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Assignment Statements: Unary Assignment
Operators
• Unary assignment operators in C-based languages
combine increment and decrement operations
with assignment
• Examples
sum = ++count (count incremented, then assigned
to sum)
sum = count++ (count assigned to sum, then
incremented
count++ (count incremented)
-count++ (count incremented then negated)
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Assignment as an Expression
• In the C-based languages, Perl, and JavaScript,
the assignment statement produces a result
and can be used as an operand
while ((ch = getchar())!= EOF){…}
is carried out; the result
(assigned to ch) is used as a conditional value
for the while statement
• Disadvantage: another kind of expression side
effect
ch = getchar()
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Multiple Assignments
• Perl, Ruby, and Lua allow multiple-target
multiple-source assignments
($first, $second, $third) = (20, 30, 40);
Also, the following is legal and performs an interchange:
($first, $second) = ($second, $first);
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Assignment in Functional Languages
• Identifiers in functional languages are only names
of values
• ML
– Names are bound to values with val
val fruit = apples + oranges;
- If another val for fruit follows, it is a new and different
name
• F#
– F#’s let is like ML’s val, except let also creates a new
scope
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Mixed-Mode Assignment
• Assignment statements can also be mixedmode
• In Fortran, C, Perl, and C++, any numeric
type value can be assigned to any numeric
type variable
• In Java and C#, only widening assignment
coercions are done
• In Ada, there is no assignment coercion
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Summary
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Expressions
Operator precedence and associativity
Operator overloading
Mixed-type expressions
Various forms of assignment
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